Abstract
Spawning salmon deliver annual pulses of marine‐derived nutrients (MDN) to riverine ecosystems around the Pacific Rim, leading to increased growth and condition in aquatic and riparian biota. The influence of pulsed resources may last for extended periods of time when recipient food webs have effective storage mechanisms, yet few studies have tracked the seasonal persistence of MDN. With this as our goal, we sampled stream water chemistry and selected stream and riparian biota spring through fall at 18 stations (in six watersheds) that vary widely in spawner abundance and at nine stations (in three watersheds) where salmon runs were blocked by waterfalls. We then developed regression models that related dissolved nutrient concentrations and biochemical measures of MDN assimilation to localized spawner density across these 27 stations. Stream water ammonium‐N and orthophosphate‐P concentrations increased with spawner density during the summer salmon runs, but responses did not persist into the following fall. The effect of spawner density on δ15N in generalist macroinvertebrates and three independent MDN metrics (δ15N, δ34S, and ω3:ω6 fatty acids) in juvenile Dolly Varden (Salvelinus malma) was positive and similar during each season, indicating that MDN levels in biota increased with spawner abundance and were maintained for at least nine months after inputs. Delta 15N in a riparian plant, horsetail (Equisetum fluviatile), and scraper macroinvertebrates did not vary with spawner density in any season, suggesting a lack of MDN assimilation by these lower trophic levels. Our results demonstrate the ready assimilation of MDN by generalist consumers and the persistence of this pulsed subsidy in these organisms through the winter and into the next growing season.
Highlights
Organisms and material routinely move between habitats, and many of these subsidies are marked by short, intermittent periods of abundance (Polis et al 1997, Yang et al 2008). These pulsed subsidies—exemplified by events such as insect mass emergence (Williams et al 1993) and mast fruiting in plants (Ostfeld et al 1996)—are readily exploited by generalist consumers due to their ability to switch prey (Ostfeld and Keesing 2000, Sears et al 2004) and can generate bottomup perturbations that influence population dynamics and community structure across multiple trophic levels (Yang et al 2008)
Log PO4-P contained local spawner density (LSD), region, season, and LSD 3 season interaction (Table 7). Concentrations of both nutrients increased with spawner abundance during the summer spawning season, when point estimates of concentrations for both nutrients increased by approximately two orders of magnitude across the observed range in spawner density (i.e., 0–20 MT/km; Fig. 2)
Ammonium-N and orthophosphate-P concentrations increased with spawner density during the summer but not during spring or fall, indicating the presence of a dissolved nutrient spike that had largely attenuated by fall sampling
Summary
Organisms and material routinely move between habitats, and many of these subsidies are marked by short, intermittent periods of abundance (Polis et al 1997, Yang et al 2008). Annual pulses of marine-derived nutrients (MDN), conveyed to freshwater ecosystems by millions of spawning, semelparous Pacific salmon (Oncorhynchus spp.) in the form of eggs, excreta, carcasses, and emergent fry, are an ecologically important energy and nutrient subsidy to stream and riparian ecosystems (Gende et al 2002, Naiman et al 2002, Schindler et al 2003). Riparian vegetation can derive a substantial proportion of nitrogen from this pool (Bilby et al 2003, Reimchen et al 2003), which may lead to enhanced growth rates (Helfield and Naiman 2001, 2002; but see Kirchhoff 2003 and Helfield and Naiman 2003)
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